1. Field of the Invention
The present invention generally relates to the field of subsea electric power cables. More specifically, it relates to a coaxial dynamic riser and feeder cable for direct electric heating (DEH) of an oil or condensate pipeline and a termination of the coaxial feeder cable to the pipeline.
2. Description of the Related Art
The temperature of the oil or condensate in the underground reservoirs is typically about 90° C. The oil or condensate well stream contains several liquid substances that freeze when the temperature drops. This is a problem when the pipes are cooled in seawater, particularly during a shutdown of production, which causes the flow in the line to be impeded or even blocked due to the formation of hydrates or wax plugs. To solve this problem chemical treatments are mainly used. However, this method has considerable operational costs and presents a risk to the environment should a leakage occur.
As an alternative to chemical treatment, electric heating has been suggested. Three methods may be used: i) electric heating cables, ii) electromagnetic induction heating, or iii) direct electric heating of the pipeline. The first alternative is found to be rather inefficient, and the second very expensive.
The direct heating system is based on the fact that an electric current in a metal conductor generates heat due to ohmic loss. The power supply is then connected directly to the electrically insulated steel pipe.
For heating of oil or gas pipelines in order to prevent hydrate and ice formation on the pipeline walls, the present applicant has developed a direct electrical heating system that is described, inter alia, in British patent specification No. 2.373.321. For current supply to such a heating system a common practice is to install a current supply cable as a so called “piggyback” cable, which is traditionally made simultaneously with the laying of the pipeline. More specifically such a single-core cable is strapped to the pipeline during installation thereof. The return current should of course as a whole flow through the pipeline walls in order to generate the heating effect aimed at.
GB 2 341 442 A describes an example of a heating system which can be used for pipelines on the sea floor. In this system, the metallic tube of the pipeline is electrically and thermally insulated and connected to a power supply which feeds a current through the metallic tube, whereby an efficient heating is achieved with alternating current.
For the energy supply of electric devices on the seafloor subsea cables are known which contain one or several conductors with high voltage insulation, outer sheath(s), and possibly metallic armoring. The (possibly dynamic) cable connecting the topside energy supply to the (static) subsea cables may comprise a number of power cores. Only single core static cables are know to have been used in earlier direct electric heating projects.
The cable may be squeezed during installation between the pipeline and hard objects. As the pipeline expands and contracts during operation, the cable is generally subject to stretching and sliding forces against the seabed. These problems are aggravated, if the pipeline spans valleys between reefs or boulders on the seabed.
A sufficient protection can be achieved with a steel armoring if the cable carries both the feeding and the return current. The direct electric heating system, however, requires only a single conductor because the metallic tube of the pipeline is used as conductor. In this case, an alternating current causes excessive electric losses if a metal armoring is applied to the cable.
Any feasible Direct Electric Heating system will require a large current, and thus also a considerable conductor cross-section area (typically>400 mm2). It is difficult to achieve good dynamic properties with a cross section that comprises two (large) conductors, as bending properties become very dependent on the bending plane. For improving the properties in this respect, it would be possible to design and use a three-core dynamic cable, and consider one core as spare, but this solution results in dimensions, weight and cost being far from optimized.
The coaxial power cable according to the first aspect of the invention solves the dynamic problems by providing a two-conductor, high voltage, submarine cable design suitable for dynamic applications. This two-core cable is primarily intended for connection to topside supply for (single-phase) DEH (Direct Electric Heating) systems.
This coaxial cable also meets a common additional requirement regarding inclusion of non-electric elements, e.g. steel tubes or fiber optic cables, within the dynamic cable cross section. Such integration must take into consideration electromagnetic induction from the two power cores.
Electric connection between a dynamic cable and the piggyback cable and the pipeline's near end, respectively, is known to have been accomplished by means of a pair of individually armored single-core feeder cables. Replacing the single-core cables by a single, armored coaxial feeder cable eliminates the added risk of AC corrosion due to induced, circulating amour currents.
Another aspect of the present invention is related to termination of the above coaxial or concentric cables. Technical problems solved by the invention are to prevent corrosion of the outer threads and to secure enough mechanical contact between the conductor and termination head.
According to the invention, the terminal assembly comprising a concentric conductor adapter, is adapted to providing an electric connection of the electric power cable with a pipeline, where at least one cone (conically shaped metallic ring) is adapted to squeeze the concentric conductor of the electric power cable against an electric conductive part of the of the concentric conductor adapter.
A press-plate may be used to press the cone inwards and a number of threaded bolts may be used to press the plate against the cone. Electric contact between the concentric conductor and the pipeline is achieved via a concentric conductor adapter and a concentric conductor connection point.
The cone and the press plate may be made of different types of materials including steel. In some embodiments non magnetic material may be used to avoid excessive power loss due to electric currents.
To summarize, the electric power cable according to one aspect of the invention has the excellent dynamic and electromagnetic properties required to solve central problems related to such cables. The terminal assembly according to another aspect of the invention, is, compared to prior art solutions, easy to make corrosion resistant, relatively fast and easy to mount at site and cost effective.